It's been said but it's worth saying again, Craigslist. It's truly free minus about 5 minutes of your time to make the ad. I started my company in July which is really not a great time to start a lawn care business and so far I have landed 4 regular maintenance accounts, 2 larger one off jobs (landscaping, one time mows aren't in my business plan), and 2 full days worth of aerations this fall all from my craigslist ads. If I were a huge company that amount of return wouldn't support the growth I need but for a small company just starting out it's great and the growth quickly turns into an exponential thing.
Other than that whatever type of media you can get printed in bulk the cheapest that looks professional, then spend your free time handing them out and don't be afraid of getting creative. Obviously hanging them on the doors of the properties you want to service is a good idea but there are more efficient ways of getting your message out there. I will never condone breaking the law so check with authorities in regards to solicitation laws but if you're after high end most hospitals have a specific doctors parking lot, instead of houses a hundred feet or so away all their cars are 3 feet away from each other. People with money tend to spend time at country clubs etc. Finding places where your target demographic congregate and putting your message out there any way you can will provide a return.
Also don't overlook the power of a website. People use the internet for everything, get a page, make it rank on google and answer your phone. Often the first person to answer the phone gets the job. I'm still working on the website part myself but I'm getting there.